The present invention relates to a process for the production of a polyurethane cushion backing on a substrate. This invention requires an isocyanate based on polymethylene poly(phenylisocyanate) having a monomer content of less than 55%, a 2,2'- and 2,4'-methylene bis(phenylisocyanate) content of less than 3%, a functionality of less than 2.5, an isocyanate group content of 25 to 30%, and a urethane group content of from about 2 to 6%.
Various methods for the production of polyurethane cushion backing on substrates, particularly carpets, are well known and described in, for example, U.S. Pat. Nos. 4,132,817 and 4,278,482. These patents also describe different apparatus for the processes described therein.
Several patents disclose the use of general polyurethane formulations in combination with specific additives, including, for example, catalysts (U.S. Pat. Nos. 4,189,543, 4,515,846, and 4,611,044), fire suppressants (U.S. Pat. Nos. 4,397,978 and 4,435,459) and fillers (U.S. Pat. Nos. 4,296,159 and 4,525,405). There are also patents relating to the use of general polyurethane formulations in specific processes (U.S. Pat. Nos. 4,657,790 and 4,913,958), or in conjunction with specific types of textiles (U.S. Pat. No. 4,853,280).
There are also numerous patents relating to specific formulations based upon various isocyanates and specific polyol blends. For example, U.S. Pat. No. 4,696,849 discloses isocyanates with a polyol containing at least 30% primary hydroxyl groups; U.S. Pat. No. 4,853,054 discloses isocyanates having an average functionality of about 1.9 to 2.4 with a polyol having an EO content of from about 15 to 30% by weight. U.S. Pat. Nos. 5,102,714 and 5,227,409 relate to isocyanates having an average functionality of about 2.2 or less with a polyol having an average functionality of 2.2 or less, and equivalent weight of at least 500 and containing from 10 to 70 mole percent of monoalcohol.
U.S. Pat. Nos. 4,296,159, 4,696,849, and 5,104,693 all describe polyurethane compositions which are suitable for the production of polyurethane backed substrates, particularly carpets. U.S. Pat. No. 4,296,159 discloses a polyurethane comprising a high molecular weight polyether polyol, a low molecular weight polyol, and organic polyisocyanate or polyisothiocyanate, and an inorganic filler. The isocyanate used in the examples are either isocyanate prepolymers based on toluene diisocyanate, or a modified diphenylmethane diisocyanate. TDI monomer is less desirable from an industrial hygiene standpoint.
The polyurethane composition disclosed in U.S. Pat. No. 4,696,849 that is suitable for carpet backing comprises a specific high equivalent weight polyol composition, a low equivalent weight compound having 2 active hydrogen atoms, a polyisocyanate, and a catalyst. Toluene diisocyanate, 2,4'- and 4,4'-diphenylmethanediisocyanates, and the isocyanate-terminated prepolymers thereof are said to be suitable isocyanates. However, the isocyanates may only contain minor amounts of the corresponding polymeric derivatives as the average functionality of the reactive components (i.e. all the active hydrogen containing components and isocyanates) must range from 1.97 to 2.03.
U.S. Pat. No. 5,104,693 discloses polyurethane compositions which are suitable for carpet backing comprising an isocyanate-reactive material having an average equivalent weight of 1000 to 5000, a blowing agent, and a polyisocyanate wherein at least 30% by weight of the polyisocyanate is a methylene bis(phenylisocyanate) soft segment prepolymer having an NCO content of about 10 to 30% by weight. This soft segment prepolymer has an average functionality of about 1.9 to 2.5, and is prepared by reacting MDI with an organic polymer having an equivalent weight of from about 500 to 5000, preferably from 1000 to 3000.
The present invention has several advantages. These include an isocyanate containing no TDI monomer, which means the isocyanate is environmentally safer and easier to handle; and the reaction has a fast cure time and produces a polyurethane with good flexibility. This particular combination of processing and properties is important in the carpet backing industry.